The present invention relates in general to the art of rock bits used in drilling petroleum wells and the like, and, more particular, to a multi-cone rock bit and a method of its assembly that assures highly accurate gage diameter, cone axis angle, and cone axis offset.
Multiple-cone rock bits, typically three-cone, are well known and commonly used in the petroleum industry for drilling wells. These rock bits are attached at the end of a drill string and rotate with the drill string. Cutter cones of the rock bit penetrate rock by crushing, cutting, gouging and scraping the rock. The cones work on the bottom wall of the hole and the side walls of the hole. The cones mount on journals for rotation. The axis of each cone inclines at an acute angle to the rotational axis of the rock bit. It is preferred practice to have the axis of the cones offset from the axis of the rock bit. Offset is the distance the axis of each cone is from the axis of rotation of the rock bit at the closest approach of the two axes. Offset produces wall gouging at the base of the hole by the cone dragging across this wall as well as rotating over it.
It has been the practice to form rock bits in segments. In a three-cone rock bit three segments are used. Each segment has a cone. A pin end of the segment is threaded for attachment to a drill string. The segments are joined together by welding at longitudinal faying surfaces at the pin end of the rock bit.
The cutter cones of rock bits have teeth or inserts that mesh with elements of or are received in recesses of adjacent cones so that the entire hole bottom faced by the cones experience the penetrating action of the teeth. This requires accurate alignment of the cones so that they do not interfere with each other during rotation.
The diameter of a hole produced by a rock bit is called the gage diameter. This diameter must be held to close tolerance. If the diameter is too small, it may not be possible to install casing in the hole without enlarging it, and this enlargement of course requires time and expense. In some applications the bit must pass through casings with a minimum diameter. An over-sized bit would interfere with the wall of the casing. In other operations it is necessary for a bit to pass through an already fabricated well bore to get to the area being drilled. It is obviously highly desirable to pass the bit through the preformed hole without encountering the bore wall.
Heat treating and machining develop dimensional part-to-part variations that cannot be controlled sufficiently so that the segments of a rock bit go together without adjustment. There has been resulting difficulty in maintaining gage diameter, journal angle, and offset within required tolerances. At least in some cases, the assembly of rock bits has developed into an art. In the usual method, each segment is completely finished. The segments are then preliminarily assembled and the gage diameter of the assembly determined by a guage ring. The assembler can shim between the faying surfaces to increase the gage diameter. Alternatively, a modest amount of scissoring or skewing of the segments with respect to each other can effect changes in gage diameter. Sometimes both techniques are employed. After acceptable gage diameter has been achieved, the segments are welded together.